The present invention relates to an active matrix type liquid crystal display apparatus, in which an electric field is applied, mainly parallel to the substrates, and in which a broad visual field and a large numerical aperture are realized.
In a conventional liquid crystal display apparatus, the electrodes for driving the liquid crystal layer are formed on the surface of two substrates, and transparent electrodes are used, the electrodes facing each other. This is because what is used is a twisted nematic display method (TN method) in which the liquid crystal layer is driven by applying an electric field in a direction vertical to the surface of the substrate. On the other hand, a display method in which an electric field is applied in a direction almost parallel to the surface of the substrate using comb-type electrodes has been disclosed in the Japanese Patent publication No. 63-21907 (1988) and U.S. Pat. No. 4,345,249. In this case, the electrodes are not necessarily selected to be transparent, but non-transparent and metallic electrodes with a higher electric conductivity are used. However, in the above-mentioned prior art, for the display method in which the electric field is applied in a direction substantially parallel to the surface of the electrode, which is designated as a “horizontal electric field method”, specifically used in the active matrix drive mode, the material property required to realize a high numerical aperture is not described in detail.
In the conventional active matrix type liquid crystal display apparatus, typically using the twisted nematic method, a transparent electrode is used. It is, therefore, possible to make the numerical aperture relatively wide, which is the area that the light per unit pixel passes through. However, in the horizontal electric field method, an opaque metal electrode is used. It is, therefore, impossible to realize a large numerical aperture. In this regard, there is an essential problem in that the opaque portion of the electrode can not be used as part of the area that light passes through. Further, the brightness of the display apparatus also depends upon the magnitude of the numerical aperture. Even if the intensity of the background light is increased in an effort to alleviate this problem, the problem arises that the power consumption is increased extremely.
Accordingly, in order to realize a large numerical aperture when employing the horizontal electric field method, it is necessary to enlarge the gap between the electrodes. However a new problem arises due to the enlargement of the gap. First, a disturbance of the orientation occurs due to static electricity, because the volume of the liquid crystal decreases further. Generally, the electrode in the horizontal electric field method is different in configuration from the one in the other type. Therefore, the volume of the liquid crystal is not so large. If the volume becomes smaller by enlarging the gap between the electrodes, the liquid crystal is susceptible to the effects of static electricity. As a result, the disturbance of the orientation due to static electricity increases. Secondly, if the gap between the electrodes becomes large, a problem arises in that the display method driven by the electric field requires a large drive voltage.